Editing IPCC:AR6/SROCC/Chapter-3 (section)

===== 3.2.3.2.1 Plankton and pelagic primary production =====

Changes in column-integrated phytoplankton biomass for the Southern Ocean are coupled with changes in the spatial extent of ice-free waters, suggesting little overall change in biomass per area at the circumpolar scale (Behrenfeld et al., 2016 <sup>[[#fn:r698|698]]</sup> ). Arrigo et al. (2008) <sup>[[#fn:r699|699]]</sup> also report no overall trend in remotely-sensed column-integrated primary production south of 50 ° S from 1998 to 2006. At a regional scale, local-scale forcings (e.g., retreating glaciers, topographically steered circulation and sea ice duration) and associated changes in stratification are key determinants of phytoplankton bloom dynamics at coastal stations on the West Antarctic Peninsula (Venables et al., 2013 <sup>[[#fn:r700|700]]</sup> ; Schofield et al., 2017 <sup>[[#fn:r701|701]]</sup> ; Kim et al., 2018 <sup>[[#fn:r702|702]]</sup> ; Schofield et al., 2018 <sup>[[#fn:r703|703]]</sup> ) ( ''medium confidence'' ). For example, a shallowing trend in mixed layer depth in the southern part of the Peninsula (as opposed to no trend in the north) associated with changes in sea ice duration over a 24-year period (from 1993 to 2017) has been linked to enhanced phytoplankton productivity (Schofield et al., 2018 <sup>[[#fn:r704|704]]</sup> ). The phenology of Southern Ocean phytoplankton blooms in this region may also be shifting to earlier in the growth season (Arrigo et al., 2017a <sup>[[#fn:r705|705]]</sup> ). However, the effect of climate change on Southern Ocean pelagic primary production is difficult to determine given that the length of time series data is insufficient (less than 30 years) to enable the climate change signature to be detected and attributed; and that, even when records are of sufficient length, data trends are often reported as being driven by climate change when they are due to a combination of climate change and variability.

Recent studies on the ecological effects of acidification in coastal waters near the Antarctic continent indicate a detrimental effect of acidification on primary production and changes to the structure and function of microbial communities (Hancock et al., 2017 <sup>[[#fn:r706|706]]</sup> ; Deppeler et al., 2018 <sup>[[#fn:r707|707]]</sup> ; Westwood et al., 2018 <sup>[[#fn:r708|708]]</sup> ) ( ''medium confidence'' ). Trimborn et al. (2017) report that Southern Ocean diatoms are more sensitive to ocean acidification and changes in irradiance than the prymnesiophyte ''Phaeocystis antarctica'' , which may have implications for biogeochemical cycling because diatoms and prymnesiophytes are generally considered key drivers of these cycles. Both laboratory manipulations and ''in situ'' experiments indicate that sea ice algae are tolerant to acidification (McMinn, 2017 <sup>[[#fn:r709|709]]</sup> ) ( ''medium confidence'' ). Model projections of trends in primary production in the Southern Ocean due to climate change from Leung et al. (2015) <sup>[[#fn:r710|710]]</sup> are summarised in Table 3.2.

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'''Table 3.2:''' Model projections of trends due to climate change driven alteration of phytoplankton properties under RCP8.5 from 2006 to 2100 across three zones of the Southern Ocean, from Leung et al. (2015) <sup>[[#fn:r711|711]]</sup> . There is ''low confidence '' in predicted zonal changes in phytoplankton biomass due to  ''low confidence '' regarding future changes in iron supply in the Southern Ocean (Hutchins and Boyd, 2016 <sup>[[#fn:r712|712]]</sup> ). Acidification was not reported as an important driver in this modelling experiment.

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{| class="wikitable"
|-
| Zonal Band
| Predicted change in phytoplankton biomass
| Drivers
| Mechanisms
|-
| 40 ° S–50 ° S
| [[File:702f6140bb33c093f79f5159ebebed41 arrowup.png]]
| Higher mean underwater irradiance
More iron supply

| Shallowing of the summertime mixed layer depth
Change in iron supply mechanism

|-
| 50 ° S–65 ° S
| [[File:afb97dd099f055cf73abda49d9421cff arrowdown.png]]
| Lower mean underwater irradiance
| Deeper summertime mixed layer depth
Decreased summertime incident radiation (increased cloud fraction)

|-
| South of 65 ° S
| [[File:702f6140bb33c093f79f5159ebebed41 arrowup.png]]
| More iron supply
Higher mean underwater irradiance

Temperature

| Melting of sea ice
Warming ocean

|}
<!-- END TABLE -->

Previously reported declines in Antarctic krill abundance in the South Atlantic Sector (Atkinson et al., 2004 <sup>[[#fn:r725|725]]</sup> ) cited in WGII AR5 (Larsen et al., 2014 <sup>[[#fn:r726|726]]</sup> ) may not represent a long-term, climate driven, regional-scale decline (Fielding et al., 2014 <sup>[[#fn:r727|727]]</sup> ; Kinzey et al., 2015 <sup>[[#fn:r728|728]]</sup> ; Steinberg et al., 2015 <sup>[[#fn:r729|729]]</sup> ; Cox et al., 2018 <sup>[[#fn:r730|730]]</sup> ) ( ''medium confidence'' ) but could reflect a sudden, discontinuous change following an episodic period of anomalous peak abundance for this species (Loeb and Santora, 2015 <sup>[[#fn:r731|731]]</sup> ) ( ''low confidence'' ). Recent analyses have not detected trends in long-term krill abundance in the South Atlantic Sector in acoustic surveys (Fielding et al., 2014 <sup>[[#fn:r732|732]]</sup> ; Kinzey et al., 2015 <sup>[[#fn:r733|733]]</sup> ), net-based surveys (Steinberg et al., 2015 <sup>[[#fn:r734|734]]</sup> ) or reanalysis of historical data (Cox et al., 2018 <sup>[[#fn:r735|735]]</sup> ). Nevertheless, the spatial distribution and size composition of Antarctic krill may already have changed in association with change in the sea ice environment (Atkinson et al., 2019 <sup>[[#fn:r736|736]]</sup> ) ( ''medium confidence'' ) and may result in different regional trends in numerical krill abundance (Cox et al., 2018 <sup>[[#fn:r737|737]]</sup> ; Atkinson et al., 2019 <sup>[[#fn:r738|738]]</sup> ) ( ''medium confidence'' ).

The distribution of Antarctic krill is expected to change under future climate change because of changes in the location of the optimum conditions for growth and recruitment (Melbourne-Thomas et al., 2016 <sup>[[#fn:r739|739]]</sup> ; Piñones and Fedorov, 2016 <sup>[[#fn:r740|740]]</sup> ; Meyer et al., 2017 <sup>[[#fn:r741|741]]</sup> ; Murphy et al., 2017 <sup>[[#fn:r742|742]]</sup> ; Klein et al., 2018 <sup>[[#fn:r743|743]]</sup> ). The optimum conditions for krill are predicted to move southwards, with the decreases most apparent in the areas with the most rapid warming (Hill et al., 2013 <sup>[[#fn:r744|744]]</sup> ; Piñones and Fedorov, 2016 <sup>[[#fn:r745|745]]</sup> ) (Section 3.2.1.2.1) ( ''medium confidence'' ). The greatest projected reductions in krill due to the effects of warming and ocean acidification are predicted for the southwest Atlantic/Weddell Sea region (Kawaguchi et al., 2013 <sup>[[#fn:r746|746]]</sup> ; Piñones and Fedorov, 2016 <sup>[[#fn:r747|747]]</sup> ) ( ''low confidence'' ), which is the area of highest current krill concentrations, contains important foraging grounds for krill predators, and is also the main area of operation of the krill fishery. Modelled effects of warming on krill growth in the Scotia Sea and northern Antarctic Peninsula (AP) region resulted in reductions in total krill biomass under both RCP2.6 and RCP8.5 (Klein et al., 2018 <sup>[[#fn:r748|748]]</sup> ). Projections from a food web model for the West Antarctic Peninsula under simple scenarios for change in open water and sea ice-associated primary production from 2010 to 2050 (6, 15, and 41% increases in phytoplankton production with equivalent percentage decreases in ice algal production) indicate a decline in krill biomass with contemporaneous increases in the biomass of gelatinous salps (Suprenand and Ainsworth, 2017 <sup>[[#fn:r749|749]]</sup> ).

Current understanding of climate change effects on Southern Ocean zooplankton is largely based on observations and predictions from the South Atlantic and the West Antarctic Peninsula. Comparison of the mesozooplankton community in the southwestern Atlantic Sector between 1926–1938 and 1996–2013 showed no evidence of change despite surface ocean warming (Tarling et al., 2018 <sup>[[#fn:r713|713]]</sup> ). These results suggest that predictions of distributional shifts based on temperature niches may not reflect the actual levels of thermal resilience of key taxa. Sub-decadal cycles of macrozooplankton community composition adjacent to the West Antarctic Peninsula are strongly linked to climate indices, with evidence of increasing abundance for some species over the period from 1993 to 2013 (Steinberg et al., 2015 <sup>[[#fn:r714|714]]</sup> ). Pteropods are vulnerable to the effects of acidification, and new evidence indicates that eggs released at high CO 2 concentrations lack resilience to ocean acidification in the Scotia Sea region (Manno et al., 2016 <sup>[[#fn:r715|715]]</sup> ) ( ''medium confidence'' ).

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